Sometimes, it just doesn’t work.
Sometimes, no matter how deeply you’ve studied a site — spending a whole year mapping out seasonal changes, the path of the sun, rainfall and water flows, etc., etc. — and no matter how much you’ve studied soil conditions and plant possibilities in order to find the best possible solution, it just doesn’t work.
Sometimes, plants that, on paper, should thrive in precisely the conditions you’ve identified, die with (what seems like) no good reason.
This can be really disappointing and deflating — especially if you’ve spent quite a lot on a particular plant(!).